Begin with the room
Stand where the plant will live and watch the room on two ordinary days. Note when the glass admits direct sun, and where nearby walls or trees cast shade.
Consider distance
A bright-looking room is not equally bright everywhere. A shelf two metres from the window can receive much less usable light than a sill just beside it.
Make one adjustment
Move a struggling plant a modest distance rather than changing its soil, water and location together. Compare the next few leaves with the older growth.
Notice seasonal movement
The same window receives light at different angles through the year. A place that feels gentle in winter may become intense in summer.